A printing method is regarded as a promising method for forming a wiring pattern, in view of its favorable aspects including low energy, low cost, high throughput and on-demand production. This purpose can be realized by a method including forming a pattern by a printing method using an ink paste containing a metal element, and then imparting metallic conductivity to the printed wiring pattern.
For this purpose, a conductive paste in which silver or copper in flake form is mixed with a binder such as a thermoplastic resin or a thermosetting resin, an organic solvent, and if necessary, a curing agent, a catalyst or the like, has been used. The conductive paste is used by applying the same onto an object with a dispenser or by screen printing, and then curing the binder resin by drying at normal temperature or heating at approximately 150° C. to form a conductive coating. In the obtained conductive coating, only some of the metal particles existing therein are in physical contact with each other so as to establish conduction, and a cured resin gives rise to the strength of the electrically conductor layer and the adhesion with respect to a substrate.
However, in this conductive paste, conduction is established by physical contact of the particles, and a binder remains partly between silver particles and inhibits the contact. Therefore, the volume resistivity is, although it depends on the conditions for formation of the coating, in a range of from 10−6 Ω·m to 10−7 Ω·m, which is from 10 to 100-fold greater than the volume resistivity of metallic silver or metallic copper, namely from 16×10−9 Ω·m and 17×10−9 Ω·m, indicating a much inferior conductivity compared to a metal film. In addition, silver particles used in a conventional silver paste are in the form of a flake having a particle size of from 1 μm to 100 μm, and cannot be used for forming a wiring line having a width of not greater than the particle size of the silver particles. Consequently, the conventional silver paste is not suitable for forming a fine wiring pattern.
In order to overcome the drawbacks of a conductive paste including silver or copper, a wiring pattern formation method in which metallic nanoparticles are used has been investigated, and a method in which nanoparticles of gold or silver are used has been established (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-273205). However, use of nanoparticles of a noble metal such as gold or silver increases the price of the material itself, and the production cost of a dispersion for ultrafine printing is also increased, which inhibits a wide diffusion of the same as a general-purpose product from economic aspects. In addition, in a case of silver nanoparticles, a further problem arises in that insulation between the circuits may deteriorate due to electromigration as the wire width or the space between the wires becomes narrower.
Copper is less likely to cause electromigration and less expensive as compared with gold or silver, and is expected to serve as a material for a metal nanoparticle dispersion used for wiring formation. As a nanoparticle dispersion including copper, a dispersion in which metallic copper nanoparticles are dispersed in a dispersing agent (see, for example, Japanese Patent (JP-B) No. 3599950) and a dispersion in which copper oxide nanoparticles are dispersed in a high polarity organic solvent have been known.
Incidentally, while a conductive paste, in which a metal flake is mixed with a resin binder, can be bonded to a substrate by means of adhesion of the binder resin, a metal nanoparticle dispersion, which does not include a binder resin, has a problem in terms of adhesion with respect to a substrate.
One solution to the problem is to add a resin in order to increase adhesion. However, in this case, another problem arises in that the resin existing between metal nanoparticles may prevent mutual contact or fusion of the metal nanoparticles, thereby causing an increase in the volume resistivity of the conductor layer. In view of this, an attempt has been made in order to improve the adhesive property by modifying a resin of a substrate (see, for example, JP-A No. 2008-200557).